One of the most insidious computer viruses today is the backdoor Trojan. The name backdoor Trojan comes from a melding of metaphors. The term Trojan comes from a reference to the legendary Trojan horse that destroyed the city of Troy during a war with Greece. Like the Trojan horse the backdoor Trojan at first looks like a gift, only later does the user find that the enemy was hidden inside. Often, it is piggybacked onto a game or some other entertaining program. In this way the computer user is tricked into downloading the Trojan horse backdoor virus. Once the program is downloaded the backdoor Trojan copies itself into the Windows system directory. This effectively creates a backdoor into the system allowing whoever planted the original program, usually called an "intruder" or "cracker", to enter your system. Once they have gained access the intruder can do anything with the computer that a user sitting in front of the computer could do. E-mail addresses, passwords, bank account numbers and anything else stored on the victim's computer is accessible to the remote user, individual keystrokes can even be recorded. While all this is going on the computer shows no signs that there is any problem, no icon or window pops up to signal access by a remote user. The only way to tell that the virus backdoor may have been installed is unexplainable hard drive activity, something that the average user probably wouldn't notice.
The Trojan horse backdoor virus is comprised of two parts. The first part is the "server". This is the part of the virus that infects the system, and opens the backdoor into the computer. The second part is the "client". The client is the part installed on the intruder's computer that allows the intruder to find and access the server, thereby gaining access to the victim's computer.
Some intruder's use the backdoor Trojan purely for destructive purposes, presumably providing themselves some sort of entertainment. They may destroy data, move data, or change passwords to keep the legitimate user out of their own files. They can also delete system files, rendering the computer effectively useless. Others use their backdoor access to steal information, like bank account numbers, and copy them onto their own hard drives to be used for their own purposes or even sold to a third party at a later date.
Because it shows very little signs of existing, a virus backdoor is very hard to find and remove. The problem is complicated by other factors as well. Firstly, because the Trojan Horse Backdoor virus copies itself into the Windows system directory Windows often will not allow the user to delete the backdoor Trojan while Windows is running. Some of the regular virus software such as Norton and Symantic can find and remove Trojans while Windows is not running.
Also, there are special Trojan remover programs out there, but they can be dangerous since some Trojans are disguised as remover programs, thus compounding rather than solving the original issue.